The present invention generally relates to communications systems and, more particularly, to wireless systems, e.g., terrestrial broadcast, cellular, Wireless-Fidelity (Wi-Fi), satellite, etc.
Digital Video Broadcasting-Terrestrial (DVB-T) (e.g., see ETSI EN 300 744 V1.4.1 (2001-01), Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television), is one of the four kinds of digital television (DTV) broadcasting standards in the world, and DVB-H is a standard for handheld applications based on DVB-T (also referred to herein as DVB-T/H). DVB-T uses Orthogonal Frequency Division Multiplexing (OFDM) technology, i.e., DVB-T uses a form of a multi-carrier transmission comprising many low symbol rate sub-carriers that are orthogonal.
A DVB-T/H receiver comprises an antenna and a tuner. The antenna provides radio frequency (RF) signals to the tuner, which is tuned to a selected frequency range, or selected channel. The tuner downconverts the received RF signal in the selected channel to provide either an intermediate frequency (IF) signal or a baseband signal for further processing by the DVB-T/H receiver, e.g., to recover a television (TV) program for display to a user. Typically, a tuner performs downconversion with a mixer and a Voltage Controlled Oscillator (VCO). The VCO is an important element in the tuner. Unfortunately, the VCO is a main contributor of phase noise (PHN).
Generally, PHN is not a big problem for analog TV systems. However, for DTV systems using OFDM, the impact of PHN on receiver operation is much more significant. In particular, PHN introduces a common phase error (CPE), which causes a rotation of the signal constellation; and also creates an inter-carrier interference (ICI) term that adds to any channel noise. As a result, both CPE and ICI interfere with demodulation of the received DVB-T signal and, therefore, removal of PHN in a DVB-T/H receiver is very important.
With regard to CPE, a DVB-T receiver can estimate the CPE and correct for it by using pilots (predefined subcarriers (i.e., frequencies) having a given amplitude and phase) that are present in each OFDM symbol. In DVB-T there are two types of pilots: scattered pilots (SP) and continual pilots (CP). The continual pilots have fixed locations within OFDM symbols and are used for CPE removal.
A conventional CPE removal arrangement is shown in FIGS. 1 and 2. In DVB-T there are two modes of operation, a 2K mode—corresponding to the use of 2048 subcarriers—and an 8K mode—corresponding to the use of 8192 subcarriers. In this example, it is assumed that the receiver is operating in the 8K mode. Operation in the 2K mode is similar and not described herein. The CPE removal arrangement of FIG. 1 comprises Fast Fourier Transform (FFT) element 105, spectrum shift element 110, CPE removal element 115 and channel estimation and equalization (CHE) element 120. FFT element 105 processes a received baseband signal 104. The latter is provided by, e.g., a tuner (not shown) tuned to a selected RF channel. FFT element 105 transforms received baseband signal 104 from the time domain to the frequency domain and provides FFT output signal 106 to spectrum shift element 110. It should be noted that FFT output signal 106 represents complex signals having in-phase and quadrature components. Typically, FFT element 105 performs butterfly calculations as known in the art and provides reordered output data (8192 complex samples in an 8k mode of operation). As such, spectrum shift element 110 further processes FFT output signal 106 to rearrange, or shift, the FFT output data. In particular, spectrum shift element 110 buffers one OFDM symbol and tidies the subcarrier locations to comply with the above-mentioned DVB-T standard and also shifts the subcarriers from [0, 2π] to [−π, +π] to provide spectrum shifted signal 111. CPE removal element 115 processes spectrum shifted signal 111 to remove any CPE (described below) and provides a CPE corrected signal 116 to CHE element 120. CHE element 220 processes the CPE corrected signal 116 for (a) determining channel state information (CSI) for providing CSI signal 122; and (b) equalizing the received baseband signal to compensate for any transmission channel distortion for providing equalized signal 121. As known in the art, CSI signal 122 may be used for obtaining bit metrics for use in decoding (not shown in FIG. 1). Equalized signal 121 is further processed by the receiver to, e.g., recover content conveyed therein (audio, video, etc.) (also not shown in FIG. 1).
Turning now to FIG. 2, the operation of CPE removal element 115 is shown in more detail. CPE removal element 115 comprises: delay buffer 155, CP extractor 160, CP locations element 165, CP memory 170, complex conjugate multiplier 175, accumulator 180, phase calculator 185, phase accumulator and sin and cos calculator 190, and rotator (also referred to as a multiplier) 195. Delay buffer 155 stores one OFDM symbol in 8K mode and thus provides for a one OFDM symbol time delay for determining an estimate of the CPE. For the 8K mode of operation, the size of delay buffer 155 is 8192×2×N bits, where N is the bit length of the data and 2 represents the in-phase and quadrature components of the complex signals. The delayed symbol is applied to rotator 195 along with a CPE estimate signal 191. Rotator 195 corrects for the CPE by rotating the delayed symbol from delay buffer 155 in the opposite direction in accordance with CPE estimate signal 191 to provide CPE corrected signal 116.
In general, the arrangement shown in FIG. 2 operates such that CPE estimate signal 191 is determined from the autocorrelation of CPs occurring at different points in time.
In particular, CP extractor 160 extracts the CPs from spectrum shifted signal 111 at particular subcarriers as defined by CP locations element 165. The latter simply stores the CP locations as defined in the above-mentioned DVB-T standard for the 8K mode of operation (e.g., see Table 7, p. 29, of the above-mentioned DVB-T standard). The extracted CPs are provided both to CP memory 170 and complex conjugate multiplier 175. Memory 170 also provides a delay of one OFDM symbol. Complex conjugate multiplier 175 multiplies the complex conjugates of CPs having the same frequencies but occurring at two different points in time (i.e., neighboring OFDM symbols). The resulting products are averaged (via accumulator 180) from which a phase error is calculated (via phase calculator 185) for each OFDM symbol. Phase accumulator and sin and cos calculator 190 further accumulates the calculated phase errors for each OFDM symbol and determines an estimate of the CPE to provide CPE estimate signal 191, which is applied to rotator 195 to correct for CPE in the signal, as described above.